Core/Database: Move singleton instances into compilation units

* Fixes issues when building shared libraries
  (prevents gcc and clang from providing several instance)
* Also instantiate the database worker pools in DatabaseEnv.cpp now.

(cherry picked from commit a8fe7f6eb7)

src/server/worldserver/Main.cpp

@@ -78,9 +78,6 @@ uint32 _worldLoopCounter(0);
 uint32 _lastChangeMsTime(0);
 uint32 _maxCoreStuckTimeInMs(0);
 
-WorldDatabaseWorkerPool WorldDatabase;                      ///< Accessor to the world database
-CharacterDatabaseWorkerPool CharacterDatabase;              ///< Accessor to the character database
-LoginDatabaseWorkerPool LoginDatabase;                      ///< Accessor to the realm/login database
 Realm realm;
 
 void SignalHandler(const boost::system::error_code& error, int signalNumber);